1. Field of the Invention
The present invention relates to a light emitting device which can be used as various illuminating light sources, and particularly, to a light emitting device in which light extraction efficiency is improved.
2. Description of the Related Art
An LED lamp (light emitting device) in which an LED (light emitting diode) is used can be formed in a compact size with food electric power efficiency and the LED lamp emits bright color light with long life. Furthermore, the LED lamp has vibration-resistant property and is strong in a repeat of turn on/off. Therefore, the LED lamp is frequently used as various illuminating light sources (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2005-19663).
FIG. 4 shows a conventional LED lamp. An LED lamp 100 includes an LED substrate 101 serving as a light emitting element and a sealing resin 102 made of silicone or the like for sealing the LED substrate 101. A fluorescent material 103 is mixed in the sealing resin 102, and various colors can be developed by combination of a color of the LED substrate 101 and a color of the fluorescent material 103.
In the conventional LED lamp, there is the following problem. The fluorescent material 103 for use in a white LED has a refractive index n ranging from 1.7 to 1.8. On the other hand, the sealing resin 102 in which the fluorescent material 103 is dispersed has a refractive index n ranging from 1.4 to 1.5. For this reason, the emitted light is scattered by the fluorescent material 103, the light impinges on a wall surface of a package or the fluorescent material 103 of itself many times, and light intensity is attenuated, which results in a decrease in light extraction efficiency.
In order to suppress the scattering of the fluorescent material 103, there is a method of forming the fluorescent material 103 having particle sizes not more than 50 nm. This is because, when the fluorescent material 103 is formed in the particle sizes not more than 50 nm, the scattering is hardly generated even if the difference in refractive index is generated. There is also a method of decreasing the difference in refractive index between the sealing resin and the fluorescent material to suppress the scattering with a resin such as epoxy having the high refractive index instead of silicone. There is also a method in which the resin is not used but the fluorescent material is used by sintering the fluorescent material.
However, in the above methods, there are the following problems. That is, the highly efficient fluorescent material having the small particle size is hardly produced, the epoxy resin is easily degraded by heat or an ultraviolet ray compared with silicone, and the sintered body is hardly realized in a Sr2SiO4 material.
On the other hand, in a conventional LED lamp 110 shown in FIG. 5, usually the refractive index of the LED substrate 111 is higher than that of the sealing resin 112. As a consequence, in the light emitted from an active layer of the LED substrate 111, total reflection is generated at an interface between the LED substrate 111 and the sealing resin 112, which results in the problem that the light is confined in the LED substrate 111 to decrease the light extraction efficiency.